Add the remaining 1 tablespoon of oil to the skillet. Add the onion and garlic and cook over moderate heat, stirring occasionally, until softened, about 5 minutes. Add the remaining 1 tablespoon of the spice blend and cook, stirring, until fragrant, about 3 minutes. Add the stock and the tomatoes and their juices to the skillet and bring to a boil over high heat, scraping to release the browned bits on the bottom. Pour the mixture into the slow cooker. Cover and cook on high for 1 hour. Turn the heat to low and cook for 1 hour longer.